This invention relates to optical interference filters and, in particular, to means for measuring the refractive index of an optical film over a broad spectral range as the film is being deposited upon a substrate.
Most optical films are produced in a vacuum deposition chamber wherein the coating material is evaporated and then allowed to condense upon a substrate under closely controlled conditions. Production problems, however, are encountered in the deposition process that may adversely affect the accuracy and repeatability with which single or multilayered filters are produced. To a great extent some of these difficulties can be avoided or overcome if the index of refraction of the film can be determined over the spectral range of interest during the time that film is growing within the vacuum chamber. By being able to rapidly and accurately determine this particular property, which is sometimes referred to as the "vacuum index", the data can be used to closely control the fabrication process.
In U.S. Pat. No. 3,892,490, a method for producing multi-layered filters is described which is designed to minimize the reflectance of a lens system used in a camera. In the process, the reflectance of each coating is monitored at one preselected wavelength as the coating grows and a resultant time-based curve is compared to a second curve depicting optimum conditions. Variances between the two curves are used to provide a basis under which adjustments to the system can be made. As only one wavelength is monitored during the growth of each layer, the described procedure cannot be used to discern the index of refraction of the film over a range of wavelengths.